Benign Prostatic Hyperplasia (BPH) can cause urinary frequency, dysuria and incomplete bladder emptying. The surgical “gold standard” for treating BPH has been the transurethral electrosurgical resection of obstructing prostatic tissue. Since its introduction some 50 years ago, transurethral resection of the prostate (TURP) has become the most widely used surgical therapy for BPH. Unfortunately, TURP associates with numerous side effects.
In the past decade, laser surgery has become an alternative to TURP for BPH treatment. High power laser beam is delivered to target prostatic tissue through an optical fiber that is introduced through an endoscope or cystoscope. Surgical outcome of laser treatment depends on a number of factors, including wavelength, power, and mode of operation (e.g., continuous or pulsed).
High power (60-80 W) Nd:YAG laser with a wavelength of 1064 nm was first used for BPH treatment in early 1990s. The advantage of Nd:YAG laser surgery is the laser's excellent hemostatic effect. At the wavelength of 1064 nm, laser light is absorbed by cellular proteins and penetrates approximately 7 mm into soft tissue. When soft tissue is heated to a certain temperature, it coagulates and shrinks. Nd:YAG laser treatment of obstructive BPH in general is not as effective as TURP.
High power (60-100 W) Ho:YAG laser with a wavelength of 2140 nm can be strongly absorbed by water and can thus evaporate soft tissue effectively. Ho:YAG laser surgery is a transurethral procedure and its clinical outcome is comparable with TURP. However, Ho:YAG laser surgery takes longer surgical time than TURP. Besides, it is technically challenging and has a steep learning curve.
High power (60-80 W) frequency-doubled Nd:YAG laser has been applied for BPH treatment since late 1990s. This laser has a wavelength at 532 nm and is transparent in water but selectively absorbed in soft tissue. This laser can effectively vaporize and ablate soft tissue and concurrently achieve hemostasis. The surgical outcome with this high power frequency-doubled Nd:YAG laser is comparable with TURP while the complication is significantly reduced.
Malek et al reported in 1998 that a 60 W frequency-doubled Nd:YAG (KTP/532) laser was used for laser vaporization prostatectomy and that the laser power was delivered continuously through an optical fiber onto prostatic tissue. Malek et al conclude in the report that “high-power KTP/532 laser vaporization prostatectomy is feasible and appears to be safe and effective for quickly relieving bladder outlet obstruction due to BPH”. The report indicates a significant improvement in clinical outcome with 60 W KTP/532 laser over early 38 W KTP/532 laser. The report also indicates that the “KTP/532 laser energy was generated by a prototype Laserscope 800 series VHP (very high power) KTP/YAG laser generator delivering 60 W power continuously”. The report further indicates that the laser has a spot diameter of 1.2 mm at 2 mm from the fiber tip, which translates into a big divergent angle, i.e., a numerical aperture of about 0.3. (See Malek et al., High-Power Potassium-Titanyl-Phosphate (KTP/532) Laser Vaporization Porstatectomy: 24 Hours Later, Urology 51: 254-256, 1998, Elsevier Science Inc.)
Davenport et al have later pointed out in U.S. Pat. No. 6,554,824 that “the problem with existing 532 nm lasers used to date is that they are large, expensive, inefficient and have a highly multi-mode output beam that makes them inefficient for ablating prostate tissue”. In U.S. Pat. No. 6,554,824, Davenport et al disclose “operation of the solid-state laser in a ‘macropulsed’ mode is more efficient in inducing rapid tissue ablation than a CW laser of the same average power”. Davenport et al also disclose that “the macropulsed laser is also more efficient and has higher beam quality, with M2 values typically less than 144, than a continuous wave laser with same average output power”. Davenport et al further disclose to generate a quasi-continuous wave (CW) beam having an output power exceeding 60 W or a high beam quality laser that “the number of transverse optical modes supported by the resonator needs to be kept as low as possible”.
It is understood that operating a solid-state laser in a macropulsed mode can increase the macropulse peak power significantly and thus increase ablation efficiency. A limitation of operating a solid-state laser in a macropulsed mode is that requires higher and pulsed pump current, which leads to shorter component lifetime. Another limitation of operating a solid-state laser in a macropulsed mode is that produces a substantially higher peak power inside the laser cavity, which may lead to power damage of intracavity optics. When the diode laser is used to pump the solid-state laser, the drive macropulse can significantly reduce CW diode laser lifetime, even damage the diode laser, and shift the diode laser wavelength from its center wavelength.
It is also understood that reducing M2 is helpful to obtain higher power densities that are required for rapid and efficient vaporization of prostate tissue. A limitation of operating a high power solid-state laser at low M2 instead of high M2 is that laser efficiency can be significantly lower and thus power consumption needs to be substantially higher. High power consumption of high power solid-state laser causes a series of inconvenience and additional expenses in hospital environment. High power consumption requires a high power outlet other than a standard wall-plug outlet. High power consumption requires external water-cooling or secondary cooling loop, which leads to extra system footage and operation cost. High power laser in general is very expensive electro-optical instrument. For the same output laser power, low efficiency laser has higher cost than a high efficiency laser. Another limitation of operating a high power solid-state laser at low M2 instead of high M2 is that power density at intracavity beam waist can be extremely high and power damage becomes a severe issue to fight.
It is further understood that operating a solid-state laser at higher average output power is helpful to obtain high power densities that are required for rapid and efficient vaporization of prostate tissue. A limitation of operating a solid-state laser at a power level substantially higher than 60 W requires substantially higher power consumption, which leads to a series of inconvenience and additional expenses in hospital environment. Another limitation of operating a solid-state laser at a power level substantially higher than 60 W is that power density at intracavity beam waist can be significantly higher, which may lead to power damage of intracavity optics.
It is even further understood that operating a high optical-to-electrical efficiency solid-state surgical laser at a power level substantially higher than 60 W can significantly reduce the system cooling capacity, eliminate the external water cooling requirement. Without requirements of external water cooling and input AC electrical current higher than 25 Amps, the high optical-to-electrical efficiency solid-state surgical laser can be easily installed in a standard surgery room. This advantage can encourage more hospitals and surgeon offices to use high power lasers in BPH treatment, such increase the consumption fiber-optic delivery devices.